Extending earlier studies (see previous annual reports), a cellular model consisting of the non-tumorigenic human osteosarcoma (HOS) cell line and either v-ki-tas or activated c-Ha-ras oncogene-transformed subclone (KRIB, AD15, and AD110 lines) were used to study the role of uPA in pulmonary metastasis. KRIB cells were the most efficient in forming experimental lung metalases in nude mice, exhibiting (in culture) the highest levels of membrane-bound uPA activity (3-9 pU per 10^6 cells). In contrast, the parental non-tumorigenic HOS cells exhibited only trace uPA activity. Cells with moderate metastatic aggressiveness exhibited paramount intermediate (reduced) levels of membrane-bound uPA activity (3-9 pU per 10-6 cells). The pivotal role of uPA in the metastatic potential of KRIB cells was demonstrated with neutralizing anti-human uPA monoclonal antibodies, which resulted in a transient but highly significant reduction in the yield of pulmonary tumors. The differential uPA activity in the osteosarcoma cell lines is likely regulated, at least in part, by the balance between uPA biosynthesis and that of its inhibitor PAI-1. Elevation in uPA mRNA levels were observed in KRIB cells, indicating a pretranslational control. However, while uPA mRNA was similarly elevated in the AD110 moderate (reduced) net enzymatic activity. These studies suggest that tumor dissemination might be controlled by suppressing localized uPA-mediated proteolysis through effects on either uPA and/or PAI-1.